(1) Field of the Invention
The present invention relates to electrodeposition compositions and methods for forming coatings and more particularly to electrodeposition compositions comprising organic acid salts of electrically conductive polymers and methods for forming coatings therefrom.
(2) Description of the Prior Art
Cathodic electrodeposition utilizes a direct current to cause positively charged electrolytes in an aqueous medium to form a coating on the cathode. The electrolytes are usually polymers with basic groups in the form of primary, secondary or tertiary amines, or quaternary ammonium, sulfonium or phosphonium groups, neutralized with an organic or an inorganic acid. Common resin compositions used in cathodic electrodeposition are modified epoxy resins containing amino groups. The resins are dispersed in water by neutralizing with organic acid. Crosslinking agents that are typically blocked isocyanates are blended with the resin to effect curing of the film upon applying heat after electrodeposition.
The positively charged polymers are dispersed in an aqueous medium such that upon applying a voltage between an article having a conductive surface serving as a cathode and a counter-electrode both of which are in contact with the aqueous medium, the positively charged polymer migrates to the conductive surface. There the polymer loses its charge, becomes insoluble and forms an insulating film on the conductive surface. As the deposition progresses, the conductive surface becomes insulated which advantageously allows uniform coating over even remote areas, i.e. interior or recessed areas. However, this insulating of the conductive surface by the deposited layer also has the effect of limiting film thickness in practice to a maximum of about 35 to 50 microns.
Cathodic electrodeposition provides excellent corrosion-protective coatings for automobiles and for appliances while diminishing the usage of the volatile organic solvents required for painting. Where metals are electrocoated, pretreatment is required to obtain optimal corrosion resistance for the coating. A chrome rinse is usually used as a pretreatment. Chromium, however, is considered to have a high toxicity hazard and, therefore, an electrocoating method that does not require a chrome pretreatment would be desirable.
International application WO 93/14166 disclosed an anti-corrosive paint comprising a binder and an electrically conductive polymer that according to the disclosure could be applied by electrodeposition. The binders disclosed included epoxy resins with crosslinkers and electrically conductive polymers including polyaniline doped with an organic acid. Electrodeposition was mentioned as one of the possible ways of applying the paint. The disclosure indicated that negatively charged paint particles can be applied in an anodic deposition process (page 44 of the WO 93/14166 application). However, such a process is inapplicable to the deposition of the positively charged polyelectrolyte of a polyaniline salt. Therefore, this reference teaches an unworkable method for deposition.
U.S. Pat. No. 5,128,396 disclosed a coating composition comprising a film forming binder such as epoxide resin with polyamide along with an amine salt of a ketoacid. One of the amines disclosed was aniline. Addition of the amine salt of the ketoacid provided an improved coating composition that could be applied by electrodeposition. This reference, however, did not disclose the addition of an organic acid salt of a polymer base. Furthermore, it has been the general view in the art of electrodeposition that low molecular weight species are undesirable in electrocoatings because they can cause film distortion and rupturing (Wismer et al., J Coatings Technol 54:35-44, 1982).
Thus, it would be desirable in the art of electrodeposition to provide an electrocoating composition and method that confer excellent corrosion inhibition upon the coating and that can be electrodeposited without requiring a chromium pretreatment of the metal. It would further be desirable to provide a component of the electrodeposited coating that could allow the film to be electrically conductive. Such a coating would passivate anodic sites on a metal surface and provide optimal protection against corrosion. If a base coat were electrically conductive, it would allow application of a second and subsequent coats by electrodeposition to achieve a greater thickness than can be obtained with a single coating or to obtain multiple layers of differing composition. Thus, it would be desirable to be able to readily electrically deposit a coating having a thickness greater than 35 to 50 microns and having multiple layers of the same or differing composition.